The backbone transmission system is required to further increase its capacity with the amount of information (traffic) in the Internet increasing. The technology for increasing the capacity includes an optical transmission technology using a plurality of channels such as an optical multilevel modulation system and a polarization multiplexing system. It has become more important to improve receiving sensitivity with the capacity increasing, and a coherent optical transmission technology in which the receiving sensitivity is very high has received a lot of attention. In the coherent optical transmission technology, AC (Alternating Current) signal components are received which have been amplified by mixing signal light and local oscillator (LO) light in a coherent optical receiver. At that time, the larger amplifying action works on the signal light as the optical output of the local oscillation (LO) light becomes larger, and therefore receiving characteristics with a high S/N (Signal/Noise) ratio can be obtained by inputting the larger local oscillation (LO) light compared to the signal light.
Patent literature 1 describes an example of such a coherent optical receiver compatible with a plurality of channels. A coherent optical receiver described in patent literature 1 includes an optical signal processing unit performing signal processing on input optical signals, separating the signals and outputting separated signals from a plurality of output ports; and a photoelectric conversion unit performing photoelectric conversion on the optical signal output from each output port and outputting the result. It further includes a plurality of output terminals having a space wider than a space between output sections of the photoelectric conversion unit; and an electric wiring unit connecting a plurality of electric signals output from the photoelectric conversion unit to a plurality of output terminals. Here, the photoelectric conversion unit is separated into a plurality of units and disposed on the center line of an output terminal of a connection destination. The optical signal processing unit has an optical expansion section where optical wiring is laid so as to be photo-coupled to photoelectric conversion units where the outputs from the plurality of output ports are separated into a plurality of ports and arranged.
The configuration enables the photoelectric conversion unit disposed separately to be disposed so that the distance to the output terminal may become shorter. They say, therefore, that it becomes possible to suppress the deterioration of output electric signal levels of the related coherent optical receiver having multi-channel outputs.